I just got off the phone with a SVP at a national BPO/Appraisal company we work for.  Government agencies and national lenders are "ghost shopping" their approved appraisers.

Here is apparently how it works.

Jim the Appraiser conducts a series of valuations and they seem a little high.  Brokers are hired to do BPO's to confirm value.  If there is a pattern of pushing the value, at a minimum the appraiser looses the client. 

It has been hinted that brokers are also being used, through some national BPO compaines, to build evidence against appraisers where complaints have been filed.

I suspect this has been going on for some time, but confirmation is now in.